This invention relates to a semi-permeable asymmetric gas separation membrane which possesses improved resistance to thermal compaction or aging. The invention further relates to processes for making and using such a membrane.
In various industries, it is necessary or desirable to separate one gaseous component from other gaseous components in a gas mixture. Processes used to perform such separations include cryogenics, pressure swing adsorption, and membrane separations.
Membranes have been used to recover, remove, isolate, or separate a variety of gases, including hydrogen, helium, oxygen, nitrogen, carbon monoxide, carbon dioxide, water vapor, hydrogen sulfide, ammonia, and/or light hydrocarbons, from a mixture of gases. Applications of particular interest include the separation of air into an enriched oxygen stream, which is useful, for example, for increasing the efficiency of fermentation processes and for enhancing combustion processes, and an enriched nitrogen stream, which is useful, for example, for inert padding of flammable fluids and for increasing food storage times. Other applications of interest include the separation of hydrogen or helium from gas mixtures containing gases such as nitrogen, carbon monoxide, carbon dioxide, and/or light hydrocarbons in addition to hydrogen or helium. For example, the separation and recovery of hydrogen or helium for recycle is often necessary in various hydrocracker, hydrotreater, and catalytic cracking processes used in the oil refinery industry. Membranes can be used to achieve such separations.
Such membrane separations are based on the relative permeabilities of two or more gaseous components through the membrane. To separate a gas mixture into two portions, one richer and one leaner in at least one gaseous component, the mixture is brought into contact with one side of a semi-permeable membrane through which at least one of the gaseous components selectively permeates. A gaseous component which selectively permeates through the membrane passes through the membrane more rapidly than at least one other gaseous component of the gas mixture. The gas mixture is thereby separated into a stream which is enriched in the selectively permeating gaseous component or components and a stream which is depleted in the selectively permeating gaseous component or components. The stream which is depleted in the selectively permeating gaseous component or components is enriched in the relatively non-permeating gaseous component or components. A relatively non-permeating gaseous component permeates more slowly through the membrane than at least one other gaseous component of the gas mixture. An appropriate membrane material is chosen for the gas mixture so that some degree of separation of the gas mixture can be achieved.
Membranes for gas separation have been fabricated from a wide variety of polymeric materials, including cellulose esters, aromatic polyimides, polyaramides, polysulfones, polyethersulfones, polyphenylene oxides, polyesters, polycarbonates, and polyestercarbonates. An ideal gas separation membrane is characterized by the ability to operate under high temperatures and/or pressures while possessing a high gas separation factor (selectivity) and high gas permeability. The problem is finding membrane materials which possess all the desired characteristics. Polymers possessing high gas separation factors generally have low gas permeabilities, while those polymers possessing high gas permeabilities generally have low gas separation factors. In the past, a choice between a high separation factor and a high gas permeability has been unavoidably necessary.
Many of the membrane materials previously used exhibit poor separation performance at high operating temperatures and/or pressures. Furthermore, many asymmetric membranes previously used suffer from the disadvantage of decreasing separation performance over time, especially when exposed to or used under conditions of high temperature and/or pressure. In particular, the phenomenon of thermal compaction has been a significant problem experienced with many conventional asymmetric gas separation membranes. Thermal compaction, also sometimes described as a thermal aging process, results from exposure of the asymmetric membrane structure to heat over a period of time. Such exposure frequently brings about a partial collapse of pores, channels, and voids present in the asymmetric membrane porous support structure, resulting in a loss of pore surface area, particularly the surface area of smaller pores in the range of from about 10 Angstroms to about 500 Angstroms, which causes the overall asymmetric membrane structure to become more dense or compact. Such compacted membranes generally exhibit a significant loss in gas flux, as well as an accompanying increase in gas selectivity. The higher gas selectivity of the compacted membrane generally does not make up for the greatly reduced gas flux, resulting in lower productivity, a less efficient separation of gases, and greater capital and operating costs required to achieve the desired separation.
What is needed is a membrane capable of separating a gaseous component from at least one other gaseous component in a gas mixture which possesses high selectivity, adequate gas permeability, and ability to operate under conditions of high temperature and/or pressure. Furthermore, what is also needed is a gas separation membrane with improved resistance to thermal compaction or thermal aging.